As the degree of integration of semiconductor integrated circuits has increased, circuit patterns have become finer, and the resolution has become insufficient in the case of conventionally used exposure apparatuses utilizing visible light or ultraviolet light. As is universally known, the resolution of an exposure apparatus is proportional to the numerical aperture (NA) of the transfer optical system, and is inversely proportional to the wavelength of the light that is used for exposure. Accordingly, as one means of increasing the resolution, there have been attempts to use a short-wavelength EUV light source (also called a “soft X-ray” light source) for exposure and transfer instead of visible light or ultraviolet light.
Light sources that are considered to be especially promising as EUV light generating devices used in such exposure and transfer apparatuses are laser plasma EUV light sources (these may be referred to below as “LPP (laser produced plasma)”) and discharge plasma ETV light sources.
In LPP, pulsed laser light is focused on a target material inside a vacuum vessel so that this target material is converted into a plasma, and the EUV light that is radiated from this plasma is utilized. Such devices have a brightness comparable to that of an undulator, while at the same time being compact.
Furthermore, EUV light sources using a discharge plasma such as dense plasma focus (DPF) are compact, produce a large amount of EUV light, and are inexpensive. In recent years, these light sources have attracted attention as light sources for EUV exposure apparatuses using EUV light with a wavelength of 13.5 nm.
An outline of such an EUV exposure apparatus is shown in FIG. 8. In this figure, IR1 through IR4 are reflective mirrors of an illumination optical system, and PR1 through PR4 are reflective mirrors of a projection optical system. W is a wafer, and M is a mask.
Laser light emitted from a laser light source L is focused on a target S, and X-rays are generated, from the target S by the plasma phenomenon. These X-rays are reflected by reflective mirrors C and D, and are incident on the illumination optical system as parallel X-rays. Then, the X-rays are successively reflected by the reflective mirrors IR1 through IR4 of the illumination optical system, and illuminate an illuminated region on the mask M. The X-rays reflected by the pattern formed on the mask M are successively reflected by the reflective mirrors PR1 through PR4 of the projection optical system, so that an image of the pattern is focused on the surface of the wafer W.
EUV light source utilizing an Xe plasma using Xe gas or liquefied Xe as target substance have been widely researched and developed for EUV light sources with a wavelength of 13.5 nm that is thus used in EUV exposure apparatuses (as well as both laser plasma light sources and discharge plasma light sources). The reasons for this are that a relatively high conversion efficiency (ratio of EUV light intensity obtained to input energy) is obtained, and that the problem of debris (scattered particles) tends not to occur since Xe is a gaseous material at ordinary temperatures.
However, in the case of techniques using Xe gas as a target, there are limits to how far a higher output EUV light source can be obtained, so that there is a demand for the use of other substances. In particular, Sn is known as an element that emits the same 13.5-nm EUV light as Xe. Furthermore, it is also known that if Sn is used, a conversion efficiency higher than that of Xe can be obtained.
However, since Sn is a metal (solid), the following problems are encountered:    (1) In the case of a laser plasma light source, a large amount of debris is generated when the solid Sn target is irradiated with the laser. If the Sn is supplied after being heated and converted into a vapor in order to avoid this problem, the density is reduced, so that a sufficiently high conversion efficiency cannot be obtained. Furthermore, solidification occurs in the low-temperature peripheral areas, so that there is a large amount of adhesion in these areas.    (2) In the case of a discharge plasma light source, it is difficult to supply a material in a solid state to the discharge space (the space where the plasma is generated between the electrodes). If the material is supplied after being heated and vaporized, the material solidifies in the peripheral low-temperature areas, so that large amounts of material adhere in these areas.
Accordingly, although it is known that Sn is a high-efficiency material, it has been difficult to use Sn “as is” as a target substance in EUV light sources. Consequently, various methods have been tried in order to overcome the demerit of Sn being a solid, which is a problem in cases where Sn is used.
For example, methods are known in which a water-soluble Sn salt is dissolved in water or another solvent and supplied as a target. In such methods, however, the following problem arises: namely, although water is eliminated by evaporation from the scattered debris following plasma generation, the Sn salt adheres to and contaminates the mirror surface as a solid just as before. Even if an attempt is made to remove the debris adhering to the mirror surface by heating and melting the debris, the heat resistance temperature of the multilayer film on the mirror surface is about 100° C. at the most; accordingly, it is not possible to achieve a very high temperature. Furthermore, even if a highly heat-resistant multilayer film that can withstand a temperature of several hundred degrees Celsius is used, there is some concern that the mirror substrate will not be able to withstand this temperature.
Accordingly, there has been a demand for a supply substance which can be continuously supplied as a target by producing a liquid whose main component is Sn (at room temperature or a lower temperature if possible), and in which scattered debris can also be easily liquefied and removed.